Industry experts fear the potentially “severe impact” on the fledgling service between Edinburgh and Tweedbank, at a time when Borderers are being encouraged to take the train rather than drive into the capital for the first time in 46 years.

Campaigners have already claimed ScotRail will be unable to operate a reliable service because the length of double track – where trains can pass each other – has been cut from 16 miles to 9.5 miles.

One industry source said: “You cannot run a half-hourly service and have steam trains running up and down the line. There would have to be changes to that frequency.”

There is further concern that steam trains could occupy one of the two platforms at the Tweedbank terminus for several hours while passengers tour the area, further reducing capacity on the line.

Campaigners said capacity cuts had been made to save money after the project’s cost rocketed to £350 million, while its completion will be three years behind schedule.

A Network Rail spokesman said: “Just as with other parts of the rail network, the Borders railway will be able to accommodate steam and heritage trains under special operating instructions.

“The details of the any timetable alterations required to allow these trains to run will be dictated by Transport Scotland and the successful ScotRail franchise bidder.”

A Transport Scotland spokesman said: “The steam train experience announced by the First Minister will run outwith the peak commuting period to minimise the impact on regular services.”

David Spaven, author of Waverley Route – the Life, Death and Rebirth of the Borders Railway, said: “Reducing the ScotRail train frequency to hourly in the middle of the day on Saturdays to accommodate tourist charter trains is not ideal, but the hourly ScotRail trains can be lengthened to cater for the extra demand.

“It’s very unfortunate the railway promoters and the political establishment ignored rail campaigners when they were pointing out these infrastructure constraints on tourist potential more than ten years ago. They were warned, but chose to ignore well-informed advice.”